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deduction confirmed by known facts? and it may be confidently affirmed that it is. Within the tropics, and especially near the equator, the barometer rises and falls with so much regularity, that it has been said "that the hour of the day may be inferred from its height." It rises from about 4 h.m. to 10 h.m.; falls from 10 h.m. to 4 h.a; rises again from 4 h.a. to 10 h.a.; and falls from 10 h.a. till 4 h.m. These changes are graphically represented in Figure 4.

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The circle, P, P1, P2, P3, represents a vertical section of the earth at the equator, and the surrounding elliptical envelope the atmosphere. The arrows show the direction in which the wave form is moving. There are two depressions, and two compressions, and four sections, in which the barometer is either rising or falling, according as the waves are advancing or receding.

It is worthy of notice that the sun is followed by a depression, and not by a compression-thus showing that its effect on the atmosphere is to produce the former.

The greater depression reaches its minimum about 4 h.a., followed by the greater compression, which attains its maximum about 10 h.a.

We have now seen by theory, experiment, and observation, that the general circulation is caused by a depression, followed by a compression moving westwards; and that these are found succeeding each other with great regularity within the tropics.

We are now, therefore, face to face with the inquiry, What is the origin of this depression? The following compression and the succeeding smaller and secondary depression and compression

need not at present engage our attention, as they could easily be shown to result from the primary.

As we have found that this depression follows the apparent diurnal revolution of the sun from east to west, we naturally turn our eyes to that luminary as the exciting cause, and to the rotation of the earth as originating its westward motion. This being so, we are led to ask, How does the sun cause the barometer to fall? The sun is the acknowledged centre of the two great forces, heat and attraction; to which of these therefore must we look as the agent in effecting this depression, and first of all let us examine the agency of heat.

The action of heat as an agent in altering the pressure of the atmosphere has been proved to be almost, if not altogether, inappreciable by Sir John Herschel in his work on Meteorology. One or two remarks may, however, not be out of place. It is well known that, even within the tropics where the heat is most intense, the temperature of the higher regions of the atmosphere is below the freezing point. The heat must, therefore, chiefly affect the lower strata of the air. The particles of air in immediate contact with the earth becoming heated, give place to colder ones which descend, and are heated in their turn, and this operation goes on without disturbing the equilibrium of the atmosphere, or altering its pressure, very much in the same manner as the particles of water are observed to do in the common experiment of heating water containing oak sawdust in mechanical mixture. To produce the necessary dynamical effect, the air would require to expand in very large volumes, and that might be the case if it were heated directly by the rays of the sun. No doubt air, when confined in a vessel, expands with considerable force, but in the atmosphere the particles are perfectly free to move in any direction. Indeed, the extreme mobility of the air is one of its most striking characteristics. A simple experiment may serve to illustrate the action of heat. If a flask (containing air) be fastened to the open end of the tube of a siphon barometer, so that the outer air is completely excluded, it will be found that the barometrical pressure remains very nearly constant for all ordinary temperatures. If the heat applied be considerable, the mercury may rise, but no ordinary

degree of cold will have the least effect. That is to say, we may increase the original pressure by adding heat, but cannot diminish it by withdrawing heat. The action of heat then, if appreciable, must cause a rise in the barometer-an effect contrary to that observed.

As the intensity of gravity varies at different places on the earth's surface, this apparatus suggests itself as a measurer of that intensity. Having now seen that heat is ineffectual in producing the depression, we may turn to consider the effect of attraction. Sir John Herschel, in the work before referred to, points out vapour tension as the probable effective cause, but I venture to express the opinion that it seems to be wanting in these elements of regularity in its action required by the conditions, and I am the more convinced of its inadequacy, as I hope presently to be able to show that attraction explains clearly and fully all the phenomena.

Attraction may be defined as that mutual force which bodies exert, by reason of which they have a tendency to move towards each other. As all bodies near the surface of the earth tend to move or gravitate, as it as been called, towards the centre, the attractions must meet and neutralize each other there. The tendency towards the centre must, therefore, be greatest at the surface and diminish towards the centre, and the sum of the tendencies being all towards the centre, the particles there must be in a high state of compression, so that the centre of the earth must be the densest part, gradually diminishing in density towards the surface. This agrees with observed facts, as the average density of the bodies near the surface is found to be much less than the mean density of the earth.

These considerations and others may be proved in the following

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A B D E (Fig. 5) represents the earth, C the centre, D E a diameter, and D, G, C, F and E particles of matter in D E.

The particle D is attracted by all the particles in the line D E, and therefore has a strong tendency to move towards E. Again, the particle E is attracted by all the particles in the line E D, and has, therefore, a strong tendency to move towards D. The particle C, being equally attracted on all sides, will have no tendency to move in any direction.

In D E take any particle as G. Then G is attracted towards E by all the particles in the line G E, and towards D by the particles in G D; but as G E is greater than G D, the resultant tendency of G is towards E; or if we take other lines of attraction, as LG K and M G H, it is obvious that the resultant tendency is still towards E.

In the same manner it might be shown that all the particles from D to C have a tendency to move towards E; that tendency being strongest at D, and diminishing towards C, the centre, where it vanishes.

By taking a point, F in C E, it may be similarly proved that all the particles in C E have a tendency to move towards D, the polarity being greatest at E, and diminishing towards C.

This proceeds on the assumption that the earth is of equal density throughout, but the actual variation in density will not affect the general argument.

The magnet affords a good illustration, for if we suppose D E to represent a magnet, the polarity is greatest at the poles D and E, and least at C.

Let us now direct our attention to the effect produced by the attraction of the sun upon these tendencies. It is clear (Fig. 5) that it will give all the particles in the line D E a tendency to move towards it; and, as we have seen that the tendency of the particles in C E is in the same, and that of those in D E in the reverse direction, the result will be to make the former tendency greater and the latter less. Now as the tendency of the particles in D C and E C to move towards the centre C produces the attraction of gravity at the surface, it is plain that the resultant effect of the attraction of the sun is to lessen the attraction of

gravity on that side of the earth immediately underneath it, and to increase that attraction on the further side, so that bodies on or near the surface at D will be made specifically lighter, while those at E will be made specifically heavier. But as D is nearer the sun than E the effect will be proportionally greater at the former than at the latter.

From this it follows that the effect of the sun's attraction is to cause a decrease of pressure in the atmosphere immediately underneath it, and an increase in pressure on the further side of the globe, although in a less degree. This might, I think, be experimentally verified within the tropics by means of the apparatus alluded to above, as capable of being used as a measurer of the varying intensity of gravity, making the vessel containing air as large as possible, and correcting it for the action of heat.

It is thus, I think, conclusively proved that the depression or fall of the barometer observable within the tropics is due to the sun's attractive force.

It need hardly be pointed out that its main effect on the atmosphere is not immediate, but mediately through the earth.

The sun's direct attraction on the air will obviously act in the same manner, but its effect I should suppose will be very slight. Electricity also affords a good illustration of this principle thus: Fig. 6.

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